CT and MR colonography: scanning techniques, postprocessing, and emphasis on polyp detection.

In the last decade, computed tomographic (CT) and magnetic resonance (MR) colonography, two new cross-sectional techniques for imaging of the colon, emerged. Both techniques show promising initial results in the detection of polyps equal to or greater than 1 cm in diameter in symptomatic patients. Imaging protocols are still mostly under development and prone to change. Both CT and MR colonography generate a large number of source images, which have to be read carefully for filling defects and, if intravenous contrast material is used, enhancing lesions. An important postprocessing technique is multiplanar reformatting, which allows the viewer to see potential lesions in an orientation other than that of the source images. Virtual endoscopy, a volume rendering technique that generates images from within the colon lumen, is used for problem solving. CT and MR colonography have potential advantages over colonoscopy and double-contrast barium enema examination: multiplanar capabilities, detection of enhancing lesions that make the distinction between fecal residue and true lesion possible, and ante- and retrograde virtual colonoscopy. Currently, a number of studies suggest that patients have a preference for CT colonography over colonoscopy. Patients consider bowel cleansing the most uncomfortable part of any colon examination; hence, from the acceptance point of view, fecal tagging techniques are promising. Before CT and MR colonography can be implemented in daily practice, they must show approximately the same accuracy as colonoscopy for polyp detection in both symptomatic and screening patients.